How are the devices connected to the computer? USB 1.1 devices connected to a USB 2.0 port communicate as USB 1.1 devices. Be careful, only devices that claim USB 2.0 Hi-Speed are actually fully USB 2.0 devices. Those that claim "Full Speed" are really only USB 1.1 devices (but that's covered under the USB 2.0 spec, so they can claim to be USB 2.0).

If they are USB 1.1 devices, then it's quite possible you have exceeded the bandwidth available.

The webcam is actually using 61% of the bandwitdth (USB composite device is the webcam - composite because it's a camera and a microphone in the same package). That definately makes it a USB 1.1 device.

The webcam is actually using 61% of the bandwitdth (USB composite device is the webcam - composite because it's a camera and a microphone in the same package). That definately makes it a USB 1.1 device.

However, how are the devices arranged on the ports?

i have the mouse and the guitar hero controller in the 2 front ports which plug into the mb inside and the webcam is plugged into the back of the motherboard.

would this be effecting the sound?
as i said if the webcam does decide to work then the sound stops... then if i restart i get sound but no webcam

I wouldn't have expected the webcam to affect the sound to be honest. However, ccperf's suggestion was to look at interrupts. You can do this by going to Control Panel -> System -> Hardware Tab -> Device Manager.
Once you're in Device Manager, select the View menu and choose "Resources by connection". That will bring up a list of four items. Expand the "Interrupt Request (IRQ)" section, and see if you can locate the sound card and anything else that might have the same number next to it.

You may want to try swapping other USB devices with the webcam and see if the same thing happens. Like try a joystick instead of the guitar-hero controller or a printer instead of the webcam. This will determine if the problem is the webcam or the computer.